Method and apparatus for detecting rotor position by use of magnetic field sensor pairs

ABSTRACT

The rotation position of a rotor ( 2 ) rotatable around a rotation axis ( 1 ) is determined by mounting a magnetic source ( 2.1 ) on the rotor, by providing on a stator ( 3 ) at least three sensors ( 4, 5, 6, 7 ) for measuring the magnetic field of the magnetic source and being arranged in at least two pairs ( 4/5, 6/7 ), calculating the difference of quantities measured by the two sensors of each sensor pair, calculating the ratio of the difference values of two pairs and comparing the ratio with a predetermined function of said ratio versus the rotation position. This rotation position determination is very robust against offset and sensitivity variations common to all sensors ( 4, 5, 6, 7 ) and against external magnetic fields and can easily be made robust against mechanical tolerances between rotating and stationary parts also. In a preferred embodiment, there are four sensors ( 4, 5, 6, 7 ) arranged at the corners of a square perpendicular to and symmetrical relative to the rotation axis ( 1 ). The sensors ( 4, 5, 6, 7 ) are advantageously Hall sensors integrated together with readout and calculation electronics in one die.

This application claims priority from U.S. application Ser. No.60/047,905 filed May 29, 1997.

FIELD OF THE INVENTION

The invention is in the field of contactless angle measurement. Itconcerns a method and an apparatus for detecting the rotation positionof a rotor with the aid of a magnetic source connected to the rotor andof stationary sensor means for measuring the magnetic field of themagnetic source. The method and arrangement according to the inventionare applicable e.g. for rotary switches or for rotary positiondetection.

BACKGROUND OF THE INVENTION

The measurement of rotation angle is required in various applications,such as manual electrical switches or position detection of a motor.Depending on cost and accuracy constraints, this task can beaccomplished by various methods, such as mechanical contacts, opticalencoders, or magnetic encoders. Modern integrated circuit technologyoffers the possibility to integrate magnetic sensors and their readoutand angle calculation electronics on one die. This allows embodiments ofdetectors of mechanical rotation which consist of a permanent magnetattached to the rotor and monolithically integrated sensor meansattached to a stator, at competitive cost, yet good performance. Theabsence of mechanical contact between the rotor with the magnet and thestator with the sensor means allows for hermetic encapsulation of thesensor means. This permits wear free angle measurements under harshenvironmental conditions.

The robustness of the angle measurement against mechanical tolerances,against device variation, and against external electromagnetic fields,while keeping fabrication cost low, is a major performance criterion.

SUMMARY OF THE INVENTION

An object of the invention is to show a method and to create anarrangement for detecting the rotation position of a rotor using amagnetic source connected to the rotor and stationary magnetic fieldsensor means for measuring the magnetic field of the magnetic source,which method and arrangement allow, compared to known such methods andarrangements, increased robustness against sensitivity and offsetvariations of the sensor means, against external magnetic fields andfurthermore against mechanical tolerances regarding the relativepositions of the sensor means and the magnetic source.

The inventive arrangement for contactless angle measurement comprises amagnetic source mounted to the rotating part (rotor) rotatable around arotation axis and an array of magnetic sensors mounted on thenon-rotating part (stator). The magnetic source is arranged such thatthe magnetic field has no rotational symmetry relative to the rotationaxis. The sensor means consists of at least three sensors arranged in atleast two sensor pairs, whereby each sensor may be replaced by a clusterof sensors (plurality of sensors arranged very close to each other). Thesensors, e.g. Hall sensors, are arranged in such a way that at least thetwo sensors of each sensor pair are sensitive to parallel components ofthe magnetic field. Furthermore, the sensors are such arranged thatconnecting lines each connecting two sensors of one sensor pair haveprojections in a plane perpendicular to the rotation axis which areangled relative to each other. Advantageously, the sensors of each pairare positioned in one plane perpendicular to the rotation axis, e.g. allpairs in the same plane.

According to the inventive method, the mechanical angle (rotationposition of the rotor) is determined by calculating at least one ratioof two differential signals of one sensor pair each (the differentialsignal of a sensor pair being the difference between the signals of thetwo sensors of the pair) and by comparing the calculated ratio with apredetermined (calculated or experimentally determined) function of saidratio versus the rotation angle. If instead of single sensors clustersof sensors are used it is the mean value of the sensor signals of thecluster which is used for calculating the differential signals.

This method yields an angle determination which is insensitive tovariations common to the two sensors of each pair (e.g. offset) and tosensitivity variations common to all sensors contributing to one ratio,as well as to external magnetic fields.

For reducing or even suppressing the influence of mechanicalmisalignment of the rotating magnetic source and the sensor array withrespect to each other and to the rotation axis, the magnetic source andthe sensor array are such designed and such arranged that the magneticfield component to be measured at any possible sensor location isdescribed by the product of a first order polynomial within a planarsurface substantially perpendicular to the rotation axis (magnetic fieldchanging linearly within such a planar surface, i.e. having nocurvature), and a function perpendicular to said planar surface, whichfunction is essentially the same for all sensor locations and isadvantageously well approximated by a linear function.

In addition to providing information about the angle of mechanicalrotation, the field of the magnetic source can be used to hold the rotorin place by adding a ferromagnetic yoke to the stator. Said yoke,shaping the magnetic field of the rotor, further enhances theinsensitivity of the device against mechanical misalignment.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in more detail referring to the followingFigures, wherein:

FIG. 1 shows a diagrammatic cross-sectional view of an exemplaryembodiment of the inventive arrangement for determining the rotationposition of a rotor 2 rotatable around a rotation axis 1, thearrangement comprising a rotor 2 containing a magnetic source 2.1, and astator 3 with a planar array of Hall devices 4, 5 and 6 (fourth halldevice not visible), e.g. monolithically integrated with signalconditioning and angle calculation electronics on an integrated circuit;

FIG. 2 shows an exemplary array of two sensor pairs 4/5 and 6/7 as seenalong the axis of rotation 1, wherein each sensor pair is essentiallylocated within a plane perpendicular to the axis of rotation 1 andwherein both pairs may but need not share the same plane;

FIG. 3 shows the magnetic field component parallel to the axis ofrotation (lines of equal field strength in square of 2×2 mm centeredbetween north and south pole), as generated by a bar magnet, at adistance of some 0.5 mm from the surface of the magnet, whereby theregion enclosed in the circle 30 (area of about 120 μm diameter)complies to the requirements as given for robustness against mechanicaltolerances;

FIG. 4 shows a cross section of a further exemplary embodiment of theinventive arrangement for measuring the angle of mechanical rotationabout an axis 1, the arrangement comprising a rotor 2 with two barmagnets 8 and 9 and a ferromagnetic yoke 10 and a stator 3 with an arrayof Hall sensors 4, 5, 6 and a ferromagnetic yoke 11, whereby theferromagnetic yoke 11 serves for holding the rotor 2 in place and forshaping the magnetic field in the vicinity of the sensors.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the inventive arrangement is illustrated inFIG. 1 (section along rotation axis 1). The arrangement comprises amagnetic source 2.1 attached to a rotor 2 rotating about an axis 1 andan array of e.g four magnetic field sensors attached to a stator 3(three sensors 4, 5 and 6 visible). The sensors are located within thefield of the magnetic source. The field sensors are sensitive to themagnetic field or to a particular component thereof, to be denoted asB_(⊥). The field sensors are e.g. Hall sensors being sensitive to thecomponent of the magnetic field perpendicular to their sensor plane andthey are integrated in one die preferably together with readout andangle calculation electronics.

As shown in FIG. 2 (top view), the exemplary sensor array comprises twopairs of sensors 4/5 and 6/7, preferably located within one plane, whichplane is oriented substantially perpendicular to the rotation axis 1 ofthe rotor 2. The connecting lines between the two sensors of each pairmust not all be parallel. It is possible that different sensor pairsshare one common sensor (e.g in an array with three sensors only).

The signal measured by each sensor is proportional to B⊥ at the sensorlocation. For each pair of sensors, the difference of the two sensorsignals is generated. The angle of rotation, to be denoted as Φ, iscalculated as a function of a ratio of difference signals. This methodis insensitive to multiplication of the sensor signals with a factorcommon to all sensors whose measured signals are utilized forcalculating one ratio of differences, as well as to the addition of asignal common to a pair of sensors, for instance due to sensor offset orexternal magnetic fields. An ambiguity of angle Φ of ±180° introduced byusing said method to determine the angle is eliminated by utilizing aplurality of ratios of differences to determine the angle, or byevaluating the sign of the difference signals of at least one of thesensor pairs, i.e. evaluating the difference signal as a positive or anegative value.

One preferred embodiment of a sensor array for an inventive arrangementcomprises two pairs of sensors, the sensors of each pair being locatedat opposing corners of a square which square is arranged perpendicularto and symmetrical with respect to the rotation axis 1 of the rotor 2,whereby all sensors measure parallel components of the field, e.g.components perpendicular to the plane of the square. In this particularcase, the mechanical angle Φ is given by:

Φ=arc tan{(S₇−S₆)/(S₅−S₄)}=arc cot{(S_(5−S) ₄)/(S₇−S₆)}.

More sensor pairs can be added for improved accuracy.

Low cross-sensitivity of the measured rotation angle Φ determined by theinventive method to mechanical translation of the magnetic source and/orthe sensor array with respect to each other and to the axis of rotationis achieved by designing the inventive arrangement such that in thevicinity of each possible sensor location, the magnetic field is linearwithin the plane in which the sensors are arranged, and such that ateach possible sensor location the change of the magnetic field parallelto the rotation axis is governed by substantially the same function.

The measures as indicated above for substantially suppressing the effectof translation are explained as follows: Let V_(i) denote the volume ina rotating coordinate frame attached to the magnetic source whichencloses the position of both field sensors of one sensor pair, labeledby i, for any rotational angle Φ and any mechanical displacement thatmay occur under permitted operating conditions of the angle detector.Let x and y denote rectilinear coordinates perpendicular to the axis ofrotation, z the linear coordinate parallel to the axis of rotation. Thefield component of the magnetic source measured by the sensors withinthe volume V_(i) is essentially described by a function B⊥(x,y,z)=B_(i)⁰+a_(i)·x·f_(i)(z), with constants B_(i) ⁰ and a_(i) and functionf_(i)(z) independent of x and y. The functions f_(i)(z) and f_(j)(z),associated to sensor pairs i and j whereof the ratio of differences iscalculated to determine the angle Φ are essentially equal. Mechanicaltranslation of the sensor array or the magnetic source perpendicular tothe axis of rotation results in a common mode signal of sensor pairs,which is substantially cancelled by utilizing difference signals.Mechanical translation of the sensor array or the magnetic sourceparallel to the axis of rotation (i. e. a change in distance betweenmagnetic source and sensor array) does not influence the measured angleΦ as the ratio of two differential signals remains essentially unchangedwith z due to the condition on the set of f_(i)(z) functions statedabove.

For achieving low sensitivity or even insensitivity against tilt, i.e.mechanical rotation of the magnetic source and/or the sensor array aboutan axis (axis of tilt) perpendicular to the rotation axis, the inventivearrangement is designed such that in the vicinity of each possiblesensor location, the change of the magnetic field parallel to therotation axis is governed by substantially the same linear function andthe sensors of each pair are arranged such that the connecting linesconnecting two sensors of one pair are perpendicular to the rotationaxis and are intersected by the rotation axis in their middle.

The measures as indicated above for substantially suppressing the effectof tilt are explained as follows: To first order in mechanicaldisplacement, tilt of the sensor array or the permanent magnet withrespect to the predetermined axis of rotation results in a common modesignal of sensor pairs if the sensors forming a pair are essentiallylocated on the intersection of a plane perpendicular to the axis ofrotation and the surface of a cylinder centered at the axis of tilt. Forpractical occurrences of tilt, namely play of the rotor about somebearing point or tilted mounting of the sensor array, or of the magneticsource with respect to the axis of rotation, the axis of tilt and theaxis of rotation can be assumed to intersect each other in a point.Consequently, influence of tilt on the measured angle is largelyrejected by using difference signals of sensor pairs if the sensorsforming a pair are substantially located within a plane perpendicular tothe axis of rotation, such that the axis of rotation intersects saidplane in the center of each line connecting two sensors of one pair. Thebetter the functions f_(i)(z) are approximated by their first orderTaylor expansion around the nominal positions of the sensors, the betteris the rejection of tilt on the measured angle.

In an exemplified embodiment, a homogeneously magnetized bar magnet,which has the shape of a cuboid of 2 mm×3 mm×3 mm, the first dimensionbeing the distance between the two pole faces, produces a fieldcomponent which substantially fulfills the conditions as named above foroptimum rejection of sensor offset and sensitivity variations as well asfor mechanical tolerances in a volume of (0.6 mm)³, located some 0.5 mmfrom one of the rectangular surfaces of the magnet. FIG. 3 shows thelines of equal magnetic strength of the magnetic field created by themagnet as described above at a distance of 0.5 mm from one of therectangular surfaces of the magnet. The area shown in the Figure is asquare of 2 mn (2×100 μm) in the center of the rectangular surface, thenorth/south axis being oriented top/bottom in the Figure. The middleregion of this area enclosed by the circle 30 fulfills the conditions asgiven above.

In the exemplified embodiment of the inventive arrangement the cuboidmagnet is mounted on the rotor such that the rotation axis is parallelto the pole faces of the magnet and runs through the center points oftwo opposite rectangular surfaces of the magnet and the sensors arearranged e.g. in a square which is oriented perpendicular to therotation axis, which is distanced from the rectangular surface of themagnet by approximately 0.5 mm and in which the sensors of one pair aredistanced from each other by not more than approximately 1 mm(positioned within the circle 30 indicated in FIG. 3).

FIG. 4 shows a further exemplified embodiment of the inventivearrangement. In this embodiment, the stator 3 comprises in addition tothe sensor array with visible sensors 4, 5 and 6, a ferromagnetic yoke11 having the form of a ring within which the sensors are positioned.The yoke 11 fulfills the dual purpose of holding the rotor 2 in placeand of shaping the magnetic field of the rotating magnetic source toattain a volume V for locating the sensors in which volume the measuredmagnetic field component B_(⊥) complies to the conditions stated above.Being rigidly connected to the sensor array, the yoke 11 reduces changesof the field component B_(⊥) within V due to movement of the rotor 2other than rotation about the rotation axis 1.

The magnetic source according to the embodiment of FIG. 4 consists oftwo cuboid or cylindrical magnets 8 and 9 and of a further ferromagneticyoke 10 or of a U-shaped permanent magnet, whereby this magnetic sourcehas a north and a south face arranged in substantially the same planeperpendicular to the rotation axis and substantially symmetrical to therotation axis.

What is claimed is:
 1. A method for determining the rotational positionof a rotor which is rotatable about an axis of rotation wherein therotor carries a magnetic source which creates a rotationallynon-symmetrical magnetic field relative to the axis of rotation, saidmagnetic source having two magnetic poles, and for reducing theinfluence of external magnetic fields and of sensitivity and offsetvariations of sensor means on the accuracy of the determination ofrotational position, the method comprising: providing stationary sensormeans in the form of at least three sensors arranged in at least twosensor pairs so that the sensors of each sensor pair are sensitive tosubstantially parallel components of the magnetic field and whereinconnecting lines, each connecting two sensors of one sensor pair, haveprojections lying in a plane perpendicular to the axis of rotation, theconnecting lines lying at an angle relative to each other, the sensormeans positioned at a distance from the volume between the two magneticpoles, further characterized in that the sensors of the sensor meanseach sense the field strength in a specific direction in a specificsensor position, measuring local components of the magnetic field usingstationary sensor means, calculating differences between the quantitiesmeasured by the two sensors of each sensor pair and at least one ratioof the differences of two pairs, and determining the rotational positionof the rotor by comparing the at least one ratio measured by the sensormeans with a predetermined function of the field component versus therotation position of the rotor.
 2. Method according to claim 1,characterized in that for reducing the influence of mechanicaltranslation of the stationary sensor means and/or the magnetic sourcerelative to each other or relative to the rotation axis, the magneticsource is designed such that the magnetic field comprises a volume inwhich the component to be measured by the sensors varies substantiallylinearly in a plane perpendicular to the rotation axis and according toa function parallel to the rotation axis which function is substantiallythe same in all locations within said volume and the sensors arepositioned in said volume such that each line connecting the two sensorsof one sensor pair is substantially perpendicular to the rotation axis.3. Method according to claim 2, characterized in that for reducing theinfluence of mechanical tilt of the magnetic source and/or the sensormeans relative to the rotation axis, the magnetic source is suchdesigned that within said volume the magnetic field varies substantiallylinearly in a direction parallel to the rotation axis and the sensorsare arranged such that each line connecting the two sensors of onesensor pair is intersected by the rotation axis and divided into twoequal halves.
 4. Method according to claim 3, characterized in that forpreventing ambiguity between rotational positions of the rotor differingby an angle of 180°, a plurality of ratios of differences for differentcouples of sensor pairs is calculated.
 5. Method according to claim 3,characterized in that for preventing ambiguity between rotationalpositions of the rotor differing by an angle of 180°, the step ofdetermining the rotation position includes calculating the differencesof the measuring signals and the corresponding ratios with a positive ora negative sign.
 6. An apparatus for determining the rotational positionof a rotor rotatable about an axis of rotation comprising a magneticsource mounted on said rotor, the magnetic source having two magneticpoles; a stator; sensor means carried by said stator for measuring amagnetic field created by said magnetic source, said sensor meanscomprising at least three sensors in at least two sensor pairs so thatsensors of each pair are sensitive to substantially parallel componentsof said magnetic field, and wherein lines connecting each two sensors ofeach pair lie in a plane perpendicular to said axis of rotation and atangles to each other, said sensor means positioned at a distance fromthe volume between the two magnetic poles, further characterized in thatthe sensors in the sensor means each sense the field strength in aspecific direction in a specific sensor position; and means forcalculating the differences between signals of said two sensors of eachpair and for calculating a ratio of the differences for said at leasttwo sensor pairs to determine said rotational position.
 7. Arrangementaccording to claim 6, characterized in that at least part of the sensorsare replaced by a sensor cluster consisting of a plurality of sensorsarranged close to each other and in that the means for calculatingcomprises means for calculating a mean value of the measuring signals ofthe sensors of each sensor cluster.
 8. Arrangement according to claim 7,characterized in that the two sensors of each sensor pair or the sensorsof all sensor pairs are arranged in the same plane perpendicular to therotation axis.
 9. Arrangement according to claim 8, characterized inthat the two sensors of each sensor pair are arranged such that the lineconnecting the two sensors intersects the rotation axis and is cut intotwo equal halves by the rotation axis.
 10. Arrangement according toclaim 7, characterized in that the sensor means comprises four sensorsarranged in the corners of a square, which square is orientedperpendicular and symmetrical to the rotation axis.
 11. Arrangementaccording to claim 10, characterized in that the sensors are Hallsensors.
 12. Arrangement according to claim 11, characterized in thatthe sensors are integrated together with readout and calculatingelectronics in one die.
 13. Arrangement according to claim 12,characterized in that the magnetic source is a permanent magnet havingtwo opposite pole faces and that the magnet is arranged on the rotorsuch that the rotation axis goes through the center of the magnet and isparallel to the two pole faces.
 14. Arrangement according to claim 12,characterized, in that the magnetic source comprises permanent magnetmeans with two pole faces positioned substantially in one plane and thatthe permanent magnet means is arranged on the rotor such that the polefaces are substantially perpendicular and substantially symmetrical tothe rotation axis.
 15. Arrangement according to claim 14, characterizedin that the stator further comprises a stationary, ring-shapedferromagnetic yoke and in that the sensors are arranged within thering-shaped yoke.